Refrigeration systems are used in many applications for heating and cooling a controlled environment, including homes, offices and cargo boxes of transport trucks, trains, ships or planes. Before operating a refrigeration system in a cooling or heat/defrost mode, it is desirable to determine whether the refrigeration system is in an operational condition. To determine if the refrigeration system is functioning properly, a diagnostic "pretrip" routine is executed to ensure proper operation. Pretrip is a process of testing a refrigeration system before a refrigeration system is prepared in a cooling or heating/defrost mode to ensure that the system will operate properly.
U.S. Pat. No. 5,172,561 discloses a pretrip operational method for automatically checking predetermined functions and the operating performance of a transport refrigeration unit. This system measures the (current draw from predetermined electrical components with a prime mover stationary, and compares the measured current draw of those components with an allowable range for the component. When the comparison step finds that the current draw for each component checked is in the associated allowable range, the actual speed of the internal combustion engine is determined, and compared to the actual speed of the internal combustion engine to determine if it is in an allowable range. This pretrip method employs temperature sensors which sense the temperatures of the return air, discharge air and ambient air. The system is then commanded to operate in a cooling mode, and the unit is checked for adequate cooling capacity as a function of the temperature values of the return air, discharge air, and ambient air. It is then determined if the transport refrigeration system is operating in the commanded heating mode by comparing the temperatures of the return air and discharge air. Operation of the modulation valve is determined by sensing the speed of the prime mover before and after closing of the modulation valve.
Although the pretrip routine of U.S. Pat. No. 5,172,561 attempts to verify whether the system will operate correctly, it suffers from numerous defects. For example, one disadvantage is that the pretrip routine executed is the same regardless of the desired box temperature or ambient temperature. This creates numerous problems especially when heating in warm ambients or cooling in cool ambients. Another flaw of this pretrip method is that it fails to determine the causes of specific problems within the refrigeration system, and fails to isolate particular problems within specific components of a refrigeration system.